1. Field of the Invention
The present invention relates to an optical pickup device that is capable of recording information on an optical recording medium or reading out information recorded on an optical recording medium using a light source emitting light toward a recording layer of the optical recording medium. In particular, the present invention relates to a structure of the optical pickup device that support three wavelengths and can downsize a light receiving portion of a photo detector and the photo detector itself.
2. Description of Related Art
There are many types of the optical recording media including a compact disk (hereinafter referred to as a CD), a digital versatile disk (hereinafter referred to as a DVD), a Blu-Ray Disk (hereinafter referred to as a BD) that is proposed recently as a medium having a large storage capacity, and the like. In order to read information recorded on these optical recording media or to write information on these optical recording media, an optical pickup device is used. Recently, there is being developed an optical pickup device that is capable of dealing with three types of optical recording media including the CD, the DVD and the BD.
It is general method for the optical pickup device to design an optical system so that a focal point of a light beam emitted from the light source is normally adjusted on a recording surface of the optical recording medium and a spot position of the light beam follows a track formed on the optical recording medium by obtaining a servo error signal. In such a design, for example, there is a three-beam type optical pickup device in which a diffraction grating is disposed between the light source and a objective lens so that the diffraction grating divides the light beam emitted from the light source into three beams, which are projected to the optical recording medium, thereby the tracking servo signal is obtained from light information detected from reflected light of the three beams.
However, this three-beam type optical pickup device is required to be adjusted so that each of the three light beams generated by the diffraction grating is projected to the track formed on the optical recording medium with a predetermined relationship. Therefore, supposing that one optical pickup device is capable of emitting light beams having three wavelengths for a CD, a DVD and a BD, usage of this three-beam type may increase a burden of the adjusting work. In addition, the above-mentioned adjustment will be difficult particularly for a BD because it is necessary to minimize a beam spot condensed onto the recording surface of the medium formed by the light beam emitted from the light source.
For this reason, it is not easy to adopt the above-mentioned three-beam type for the optical pickup device having three light sources of three wavelengths for a CD, a DVD and a BD. Recently, there is proposed another technique about the optical pickup in which the light emitted from the light source and reflected by the optical recording medium is diffracted by a diffraction hologram element, and the diffracted light is received by a detector so that a servo error signal including a focus error signal and a tracking error signal are obtained. For example, JP-A-2000-76688 discloses an optical pickup using the above-mentioned technique, which can support a plurality of types of optical recording media using different wavelengths by a simple structure. If this technique utilizing the diffraction hologram is adopted, it is possible to obtain a reliable servo error signal without using the above-mentioned three-beam type.
Considering this, the optical pickup device having a structure as shown in FIGS. 10A and 10B can be supposed as an optical pickup device having light sources of three wavelengths supporting a CD, a DVD and a BD. FIG. 10A is a schematic diagram showing an optical system of an optical pickup device, and FIG. 10B is a plan view showing a general structure of a detection surface of a photo detector provided to the optical pickup device. Numeral 100 is an optical pickup device, numeral 101 is a semiconductor laser for a CD, numeral 102 is a semiconductor laser for a DVD, and numeral 103 is a semiconductor laser for a BD. Note that distances between a collimator lens 105 and the light emission points of the three semiconductor lasers 101-103 are equal to each other. In addition, the semiconductor laser 101 emits a light beam having a wavelength of 780 nm, the semiconductor laser 102 emits a light beam having a wavelength of 650 nm, and the semiconductor laser 103 emits a light beam having a wavelength of 405 nm.
Numeral 104 is a dichroic prism, which leads the light beams emitted from the semiconductor lasers 101-103 to the collimator lens 105. The collimator lens 105 converts the entering light beam into parallel rays. The light beam that passed through the collimator lens 105 is reflected by a beam splitter 106 and is condensed by the objective lens 107 onto a recording surface 108a of an optical recording medium 108. Then, the light beam is reflected by the recording surface 108a, passes through the beam splitter 106, and is led to a hologram element 109. The hologram element 109 works as a diffraction grating, and the light beam that passes through the hologram element 109 is diffracted so that ±1st order light beams are generated. The ±1st order light beams pass through a condenser lens 110 and are condensed onto a light receiving portion 112 of a photo detector 111. Note that the light receiving portion 112 is divided into two portions including a light receiving portion 112a for receiving +1st order light and a light receiving portion 112b for receiving −1st order light as shown in FIG. 10B.
In this way, when an optical pickup device 100 is structured, it is possible to structure an optical pickup device easily which has light sources of three wavelengths supporting a CD, a DVD and a BD and can obtain an appropriate servo error signal. However, in this structure, a diffraction angle of the light beam that passes the hologram element 109 is changed in accordance with a wavelength of the light beam. Therefore, as shown in FIG. 10A, the diffraction angle by the hologram element 109 increases with respect to the light beam for BD (shown by a dot and dashed line), the light beam for DVD (shown by a solid line) and the light beam for CD ((shown by a broken line) in this order, so that focus positions of the light beams entering the light receiving portion 112 of the photo detector 111 are extended. For this reason, an area of the light receiving portion 112 of the photo detector 111 should be enlarged, which causes an increase of cost for manufacturing the photo detector 111 and an increase of size of the photo detector 111.